At times it is necessary to convert halftoned color data into an RGB color space, from which it may have been derived. For example, a conversion is required when a document management system is to display printed documents on an RGB display, such as the front panel of an MFP or a computer screen, and the printed document storage contains only the halftone data that may be used to print documents. This conversion can be both a conversion to RGB and a scaling down of the data from a high resolution image, i.e., the high resolution image that may have been printed, to a smaller image of lower resolution, i.e., a thumbnail image that may be shown on a display screen of the printing device.
A bitmap defines a display space and color for each pixel. The colors may be defined in different spaces, such as RGB, CMYK, and LAB. Each color may be defined as some level in a range expressed by a level definable according to bit size, e.g., 1-bit, 2-bit, 8-bit, 16-bit, and 32-bit. The arrangement of the pixels may be planar, e.g., a grayscale-like planar scheme for each of red, green and blue planes, which can be combined to generate a multiple color bitmap. The arrangement of pixels may be interleaved, where, for example in a single array, each pixel contains information pertaining to the multiple colors of the arrangement, e.g., an R level value followed by a G level of value followed by a B level of value. For the contone CMYK, the planar arrangement may comprise four grayscale-like planes. The interleaved arrangement may have a pixel defined by four constituent levels, e.g., a C level value followed by a M level value, followed by a Y level value followed by a K level value.
Color data may be expressed in a CMYK interleaved color space having few, e.g., two or three, levels of gradation due to allocated bits per color component. A conversion from CMYK to RGB may be based on lookup tables of predefined general equivalents for the CMYK data. If the CMYK data is planar, each pixel may be first converted to interleaved data and then converted to RGB via a pixel-by-pixel lookup process based on the lookup tables.
Presently, the process of displaying a halftoned image may be achieved by converting a pixel at a time to RGB. Because the possible halftoned values are limited, this pixel-by-pixel conversion can be expedited by using a lookup table with the conversions. Particularly where RGB display is much smaller than the full, i.e., printable, halftone image, some exploitation in reduction in image size for the RGB display may be made. This reduction can be performed after the conversion by standard scaling libraries that may be made available to the display. This approach involves a pixel-by-pixel conversion of the pixels of the halftone bitmap. Once the pixel-by-pixel conversion is complete, the same number of pixels are subject to a scaling process to reduce the actual number of converted pixels displayed via the display screen. Alternatively, in another method all of the pixels of the halftone bitmap are first subject to a scaling process; producing a reduced set of contone halftone pixels for conversion to RGB for display. For example, the halftoned image for printing may be scaled for RGB display by averaging multiple pixels into a much smaller contone bitmap, and then a pixel-by-pixel conversion of the contone bitmap to RGB is performed.